update_letters_known_not_in_word: Establish which letters are known to _not_ be in the target word
Description
For all items in game$guess, establishes the letters which are now
known to not be in the target word. These are present as a character vector
in game$letters_known_not_in_word in the returned object.
Usage
update_letters_known_not_in_word(game)
Value
A 'wordler' game object.